This invention relates generally to sensors, and more particularly to pressure sensors including an ionic conduction sensing mechanism.
Pressure sensing technologies, or force sensing technologies, have a broad range of applications in the medical, industrial, and consumer product arenas. For various applications, sensors desirably may have different pressure sensing characteristics such as working ranges, interfaces, operating conditions, shape, size, and materials. Although a large number of pressure sensors are commercially available, the types of flexible thin film pressure sensors have been relatively limited. Flexible thin film pressure sensors typically may be used to measure the interface pressure and pressure distribution between objects (e.g., relatively soft objects), and have certain applications in which conformal bending of the sensor to the interface may be required (e.g., seat occupancy detection in the automobile industry, tactile feedback for robots to sense and respond to environments, rehabilitation progress monitoring of a patient in the medical industry, biting force mapping in dentistry applications, or force measurement on golf club grips).
Thin film pressure sensors conventionally use sensing methods that are either resistive or capacitive (see, e.g., Ashruf, C. M. A., “Thin Flexible Pressure Sensors”, Sensor Review, 22, 322-327, 2002). The resistive sensing principle may be based on resistance change of thin film sensing elements when a compressive force or pressure is applied to the sensing elements. The detailed design of a resistive sensor may vary from one sensor to another. One example of a resistive thin film pressure sensor is FLEXIFORCE® from Tekscan Inc. FLFXIFORCE® may consist of two polymer films. One film has electrodes facing the conductive surface of the adjacent film, and both of films have conductive materials and pressure-sensitive inks. When pressure is applied to such a sensor, the contact resistance between the two adjacent polymer films changes and is detected. In contrast, capacitive thin film pressure sensors may rely on a capacitance change due to a gap distance change between two plates when pressure is applied to the sensor. Thin film pressure sensors based on either resistive or capacitive principle may be re-usable.
Another pressure sensor is a disposable thin film pressure mapping product called PRESSUREX®, which is offered by Sensor Products Inc. PRESSUREX® utilizes rupture of microcapsules encapsulated with dyes for pressure sensing (See PRESSUREX® brochure, Sensor Products Inc., at www.sensorprod.com).
Other pressure sensors may use conducting material-filled polymer to detect pressure. Conducting material-filled polymer has variable conductivity, which originates from a process known as percolative conduction. Briefly, percolative conduction involves the conducting material-filled polymers undergoing an insulating-conducting transition when the volume fraction of the conducting filler in the polymer matrix exceeds a threshold value. When conventional conducting fillers, such as carbon black and metallic powders, are used, a relatively large volume fraction is required (i.e., about 15 vol. %) to cause percolative conduction. Such a high volume fraction generally makes processing very difficult due to an increase in the viscosity of the polymer filled with conducting material. Although those conducting material filler-polymers have a high percolation threshold, a lower percolation threshold (e.g., 3 wt. %) has been reported for other conducting material filled-polymer materials, such as a carbon nanotube-filled polycarbonate used for strain sensing. (See, e.g., Barrau S. et al., Macromolecules, 36, 5187-5194, 2003 and Zhang, W. et al., Nanoscience and Nanotechnology, 6, 960-964, 2006).
It would therefore be desirable to provide additional sensors capable of sensing low pressures which also may have enhanced sensitivity for sensing low pressures.